222 
NATURE 
[Dec. 23, 1869 
optician, and consists simply in giving the eye-glass of an eye- 
piece such a motion that while the face towards the field-glass 
presents an unchanged curvature, the other face (plane) is 
slightly inclined. This is clearly equivalent tothe addition of a 
prism to the eye-glass, only there is no loss of light, as there 
would be were a separate prism added. The new eye-piece will 
serve also to correct errors in the centreing of an object-glass. 
—Professor Cayley discussed certain geometrical relations con- 
nected with the problem of determining the place of a body 
revolving round the sun, from three observations. He remarked 
that each ohservation shows that the body lics on a known line. 
If we take these three lines in space, what the problem really 
requires is, that we should determine the position of a plane 
passing throrgh the sun, and intersecting these lines so that 
a conic through the points of intersection should have the 
sin in its focus, and the areas between vectoval radii to the points 
proportional to the two observed time-intervals. Hfe then 
considered the Iccus of the orbit-pole, (1) fer an assumed 
eccentricity, (2) for an assumed period, on a_ stereographic 
projection of a portion of the sphere—eqval in extent to one 
half, but not actually a hemisphere. The /océ for poles to veal 
orbits that is, orbits having all three points on one branch— 
exhibit a singular figure, the true locus having joints d’arrét 
on the bounding-lines of the half-sphere of prc jection. In answer 
to questions by the Astrcnomer Royal, Mr. De la Rue, and Mr. 
Stone, Professor Cayley indicated that his paper was intended 
rather as a contribution to the geometry of the problem than to 
its practical solution.—-Dr. Balfour Stewart then read a paper on 
terrestrial magnetism. After rcferring to the views he has 
already published respecting the earth’s magnetic character, he 
indicated his belief that the zodiacal light is a phenomenon of 
terrestrial magnetism, Owing its existence to the magnetic effects 
produced by the upper air-currents (the counter-trades) ; these 
eifects not being recognisable, of course, in ordinary observations. 
Ie suggested also that the motion of the tidal wave might account 
for the magnetic variations, whose period corresponds to the 
lunar diurnal motion. The Astrcnomer Royal remarked on the 
complexity of the whole problem of terrestrial magnetism, which 
he characterised as hardly belonging to the class of subjects 
usually dealt with by the Socicty.—Mr. Proctor gave a brief 
sketch of a new theery of the Milky Way. He regards the 
galaxy as neither a cloven disc, as Sir W. Herschel opined, 
nora flat ring as Sir J. Herschel has suggested, but a spiral cf 
a figure which he indicated as serving to explain all the most 
striking peculiarities of the Milky Way, as seen upon the heavens. 
—Another paper by Mr. Proctor, on Great Circle Sailing, and a 
letter from Lieut. Tupman about the November meteors as 
seen in Egypt, remained unread owing to the lateness of 
the hour. 
Geological Society, December 8.—Professor T. H. Huxley, 
LL.D., F.R.S., president, in the chair. The following commu- 
nications were read :—1. ‘* Notes cn the Brachiopoda hitherto 
obtained from the Pebble-bed at Budleigh Salterton, near 
Exmouth, in Devonshire,” by Thomas Davidson, Esq., F.R.S., 
F.G.S., &c. ‘The author first described the general characters, 
and discussed the opinions that have been put forward as to the 
origin of the pebbles forming this bed. Nearly forty species 
of Brachiopoda have been obtained from them. The fossils 
contained in the pebbles have been regarded as of Lower 
Silurian age; the author considered the great majority of the 
brachiopoda to be Devonian. “The species identified with Silu- 
rian fossils are :—Zingula Lesueuri (Rouault), LZ. Rouaulti 
(Salter), and Z. //awei (Rouault). The species regarded by 
the author as undoubtedly Devonian (ze. either previously de- 
seribed from Devonian deposits or associated with such species 
in the same pebble) are twelve in number, namely :—sSpirifer 
Verneurlaii (Murch), S. macroplera (Goldf.) Adhyris budleighensis 
(sp. n.), Aérypa (reticularis ?), Rhynchonella inaurita (Sandb.), 
#. elliptica (Schnurr.), R. Vicaryi (sp. n.), and two undeter- 
mined species of RAynchondla, Streptorhynchus crenistria (Phil.) 
Productus Vicaryi (Salt.), and an undetermined Chonetes. Eight 
species occurring in the same rock, three of which have been 
doubtfully identified, are considered by the author to be probably 
Devonian. The species supposed to be determined are :—Orthis 
vedux (Barr.) and O, Berthosi (Rouault), Silurian ; and Spirifer 
octophicatus (Sow.), Devonian and Carboniferous, but possibly 
identical with the Silurian .S. e/evatus (Dalm.) The others are 
two new species of Orthis, and a new Riynchonella (2), and an 
undetermined species of Terebratula (?) and Strophomena (?). 
Finally, the author noticed fourteen species (all new, except 
Orchis pulvinata, Salt.) only known from these pebbles, but 
which were stated to possess a Devonian /acres.—Mr. Etheridge 
agreed with Mr. Davidson as to his determination of the species. 
He had, however, examined the extensive collection of Mr. 
Vicary, and, from their general facies, he was inclined to assign 
them to the Middle Devonian and Carboniferous beds. The 
attribution of the fossils to Upper Llandovery beds was founded 
on the presence of Zingula crumena ; but he thought he could 
give some clue to the locality from which the pebbles had been 
derived. It had first, from the lithological character of the 
pebbles, as well as from the fossils, been thought that they were 
of Lower Caradoc age. He himself assigned the position of the 
rocks from which the pebbles had been derived to the Hangman 
group of North Devon. At Anstis Cove, Mr. Tawney had 
found a series cf the same class of fessils in a matrix exactly 
like that of the pebbles. He had examined the spot, and there 
recognised an extension of the sandstones cf North Devon (the 
Hangman Grits) on the south ceast, end certainly, so far as 
lithological character is concerned, the rocks are the same as 
the pebbles. It did not, however, follow that the pebbles came 
from that particular district, bet probably from the denudation 
of the large tract of country of Devonian age to the north. Vhere 
are, however, Silurian species in the pebbles, and these he 
would refer to the denudation of rocks in an area mainly to the 
south of what is now the Devoncoast. The fauna at Budleigh 
Salterton is essentially British, and not French, though some 
few species are common to beth areas. The bivalves, indeed, 
were hardly known in France. On the whole, he concluded 
that the fossils in the pebbles were Devonian, with a slight ad- 
mixture of Silurian and probably Carboniferous forms, derived 
from rocks at no great distance from the spot where the pebbles 
are found.—Professor Ramsay pointed out that in conglomerates 
it might be expected to find pebbles of rocks of various ages. 
He commented on the difficulty paleontologists seemed to 
labour under in determining a fossil if it came out of a pebble 
instead of from a rock the position of which was definitely 
known. He adyerted to the statement that the beds containing 
the pebbles had been deposited in the New Red Sandstone sea, 
whereas Mr. Godwin-Austen had regarded the New Red deposits 
as formed in large inland lakes; and the local character of the 
beds supported this latter view.—Mr. Prestwich was glad that 
some other source had been suggested for the quartzite pebbles. 
He had found somewhat similar quartzites between Lisieux and 
Cherbourg, in France.—The President observed that he would 
like to see the rise of a new race of paleontologists, relying 
simply on zoological characteristics, and not on geological posi- 
tion. A considerable simplification of our classification would 
probably result.—Mr. Etheridge briefly replied. 
2. “On the relation of the Boulder-clay without Chalk of the 
North of England to the Great Chalky Boulder-clay of the 
South,” by Searles V. Wood, jun., Esq., F.G.S. The author 
described the Yorkshire glacial clays as of two kinds—the lower, 
containing chalk debris, and belonging to the uppermost member 
of the glacial series in eastern and east-central England; the 
upper containing chalk sparingly in its lower part, and gradually 
losing this upwards. On the coast the latter occurs only north 
of Flambro’. He stated that, paleeontologically, the lower and 
middle glacial deposits closely ayree with tlie crag, and are quite 
distinct from the deposits at Bridlington, which he placed im- 
mediately above the “Great Chalky Clay.” The absence ct 
chalk debris in the deposit north of Flambro’ has Leen regarded 
as evidence of a drift from north to south ; but the author stated 
that the purple clay without chalk extends over much of the 
north-eastern part of the Wolds, from the sea-level to an elevation 
of 450 feet, and that outliers cf it occur at intervals along the 
Holderness coast-section as far as Dimlington, forty-two miles 
south of the northern limit of the Wolds. In the direction of 
Flambro’ and York the clay was said to be destitute of chalk, 
which would not be the case had the Wolds formed a sea-shore 
causing a drift from the north to pass either to south-east or 
south-west. The author described the characters of the great 
chalky boulder-clay in the eastern and central counties of Eng- 
land, and maintained that the chalk found in it (equal, according 
to him, to a layer of at least 200 feet over the entire Wold) could 
only have been detached by the agency of moving ice, which he 
believed to have covered nearly the whole Wold for a long 
period. The author stated that boulders of Shap Fell granite are 
confined to the deposit of clay without chalk ; and discussed the 
means by which they could have been distributed. He ascribed 
their dispersion to the agency of floating ice during an adequate sub- 
